Conventionally, inks of color elements of four colors (process colors) of cyan (C), magenta (M), yellow (Y) and black (K) are generally used in a printing machine. Colors are outputted to the surface of a sheet of paper with these color elements visually mixed. In order to output appropriate colors, which cannot be obtained with only the process colors visually mixed, inks of color elements of colors (spot colors) such as red (R), green (G) and blue (B) excluding the process colors may be used.
In printing using such a printing machine or the like, determination on whether or not colors are reproduced by use of spot colors, or determination on which spot colors are good to be used in a case where spot colors are used, is left to a user such as an image editor or a printing company. Certainly, as the number of types of spot colors to be used is increased, the number of types of colors, which can be appropriately reproduced by a printing machine, is increased. Meanwhile, as the number of types of spot colors to be used is increased, the number of operations and costs are increased. For this reason, what is required is to appropriately reproduce colors of an image by use of a printing machine while avoiding using spot colors unnecessarily. However, experiences are required to select a color element group optimal for an image to be printed, from among various color element groups such as a color element group consisting of only the process colors, and a color element group consisting of the process colors and any additional spot color, in comprehensive consideration of color reproduction, labor, costs and the like.
With the circumstances taken into consideration, a technique for obtaining an optimal color element group has been proposed (for instance, refer to Japanese Patent Application Laid-Open Official Gazette No. 2002-247403). In the technique, a skilled user beforehand obtains optimal color element groups for images of various types of contents, and the optimal color element groups for the respective content types are registered in a database. When an image is printed, the content type of the image is inputted, and an optimal color element group for the content type is searched for in the database. Accordingly, the optimal color element group is obtained. For instance, in a case of an image of a content type of an advertisement on fresh food including meat, vegetables and the like, the colors of R and G occupy a larger portion in the image. In the technique disclosed in Japanese Patent Application Laid-Open Official Gazette No. 2002-247403, the color element group, which includes the spot colors of R and G, is automatically obtained for an image of such a content type, not depending on determination by a user.
However, in the technique disclosed in Japanese Patent Application Laid-Open Official Gazette No. 2002-247403, the color element group corresponding to the content type of an image is simply selected. This results in a problem that, even if a different optimal color element would exist among color element groups in a database, a user cannot know the existence. In addition, the technique disclosed in Japanese Patent Application Laid-Open Official Gazette No. 2002-247403 has a problem that a user cannot know how much an adopted color element group is appropriate to an image to be outputted.